The channels offered by the physical layer to Layer 2 for data transport between peer L1 entities are denoted as transport channels in the 3GPP specification. Different types of transport channels are defined by how and with what characteristics data is transferred on the physical layer. A dedicated transport channel carries information from logical dedicated traffic channels or dedicated control channels. Several different logical channels (and services) can use the same transport channel, and several different transport channels can be multiplexed to one physical channel. The transport block lengths, transmission time intervals (TTI), interleaving depth, forward error correction (FEC) encoding/decoding, and so forth can be defined separately for each transport channel. The physical layer adds a cyclic redundancy check (CRC) for each transport block.
In a WCDMA system outer loop power control is used to maintain communications at the desired quality level by setting the signal-to-interference ratio (SIR) target for the fast, closed loop power control. In different environments, and for different services, different SIR targets are needed to obtain the same received quality. The SIR target value is adjusted according to an autonomous function in the mobile station in order to achieve the same measured quality as the quality target set by the Universal Terrestrial Radio Access Network (UTRAN). The quality targets can be set based on the transport channel block error rate (BLER) values for the transport channels, as signaled to the mobile station by the UTRAN. The UTRAN does not necessarily set a quality target for all transport channels.
Note that outer loop power control and fast, closed loop power control can exist simultaneously in both the uplink and downlink directions. In the WCDMA system of most interest to this invention both are present, and the mobile station is defined as the entity for controlling the open loop power control for the purpose of setting and adjusting the downlink SIR target for a network-specified link quality is met for each transport channel (e.g., for a specified link quality based on some specified value of BLER).
Further reference with regard to power control in general, and outer loop power control in particular, can be made to “WCDMA for UMTS”, by Harri Holma and Antti Toskala, Wiley & Sons, ISBN 0 471 720518, pages 34–37 and 196–203, incorporated by reference herein.
It is noted that in the 3GPP WCDMA system there can be periods of time when no data is transmitted on a transport channel. These periods can be referred to as periods of Discontinuous Transmission, or DTX. This is a problematic situation for the downlink outer loop power control function of the mobile station, as the received signal quality cannot be determined, resulting in the occurrence of wasted capacity or unnecessary errors.
That is, a problem is created if all transport channels that have a BLER target are in a state where no data is transmitted, as the BLER (quality) of these transport channels cannot be measured and used by the mobile station to determine the SIR target that is required to maintain the BLER at the specified level. As a result, when the data transmission once more begins either unnecessary errors are generated (the SIR target is set too low) or capacity is wasted (the SIR target is set too high), if the environment has changed during the time when data was not transmitted, and the SIR target was not updated to the level needed to maintain the BLER at the required level.
Reference can be made in this regard to FIG. 1, where the top trace shows the intervals when data is and is not transmitted relative to the variable SIR target and the transport channel BLER target. Note that during the time that the data is not transmitted the SIR target must be increased in order to maintain the required BLER, but no data is present for the mobile station to use in updating the SIR target. As a result, when the data is again present the SIR target is actually too low, it being assumed that environmental conditions have changed during the time that the data was not transmitted.
While it may appear that one could deal with this problem in a simplistic fashion by maintaining the SIR target constant when no data is transmitted, this approach would be undesirable, as the required SIR target value may change during the time when no data is transmitted (during DTX).
A need thus exists for an outer loop power control technique that follows the changes of the environment during DTX, enabling the SIR target value to be maintained at approximately the required level. Prior to this invention, this need was not fulfilled.